CN111023997A - Engine blade measuring method - Google Patents
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G01B11/00—Measuring arrangements characterised by the use of optical techniques
- G01B11/24—Measuring arrangements characterised by the use of optical techniques for measuring contours or curvatures
- G01B11/25—Measuring arrangements characterised by the use of optical techniques for measuring contours or curvatures by projecting a pattern, e.g. one or more lines, moiré fringes on the object
- G01B11/254—Projection of a pattern, viewing through a pattern, e.g. moiré
Abstract
The invention discloses a method for measuring an engine blade, which comprises the following steps: s1, constructing a measurement system based on structured light and acquiring a fringe pattern obtained by scanning; s2, carrying out two-dimensional S transformation on the fringe pattern to obtain a two-dimensional S transformation coefficient; s3, extracting two-dimensional S transformation coefficients at different frequencies at the same position, and recombining the two-dimensional S transformation coefficients into a matrix to obtain the frequency spectrum distribution at the position; s4, acquiring the distribution of the scheduling values of the whole fringe pattern according to the frequency spectrum distribution of each position; s5, reconstructing a three-dimensional surface shape of the engine blade according to the mapping relation between the distribution of the scheduling values and the height values of the whole fringe pattern; and S6, measuring the engine blade by detecting the three-dimensional surface shape of the engine blade. The method can avoid the problems of shadow and shielding, also avoid the phenomenon of possible discontinuity in the phase deployment process, and can realize accurate measurement of the surface shape of the blade of the aero-engine.
Description
Technical Field
The invention relates to the field of engine maintenance, in particular to a method for measuring an engine blade.
Background
In the aviation industry, defect detection of engine turbine blades plays an important role in ensuring aircraft safety and preventing major accidents. The defects of the turbine blade of the engine mainly comprise:
1. holes:
①, shrinkage cavity, wherein the shape is irregular, the size is bigger, the gray value is smaller, the difference with the gray value of the target entity is obvious, the outline of the fibrous shrinkage cavity with clear outline is dendritic, but the shape on the ICT slice is fuzzy, the gray value is smaller than the gray value of the target entity, the spongy shrinkage cavity can be seen from the image, the spongy shrinkage cavity is in a cloud shape with loose shape, the gray value is smaller than the gray value of the target entity, and the outline is not clear;
② air holes, isolated or grouped round, oval and pear-shaped dark spots, smooth contour and small gray value.
2. Impurities inclusion:
the high density inclusion composed of dense oxide skin, etc. has higher gray value than that of the target entity, is easy to be identified by naked eyes, has a shape of small granular or flaky image, and has a clear outline. And the other type of inclusion is low-density inclusion, and if the density of the inclusion is low and the gray value of the inclusion is very close to the gray value of the defect of the hole on the slice, the defect can be separated by detecting the defect of the hole. When the density of the inclusion is connected with the density of the target entity, the gray value of the inclusion defect is not greatly different from the gray value of the target entity, and the outline is fuzzy.
3. Loose type:
due to very fine, irregular pores or regions of fine, shrivelled clusters within or on the casting. The judgment can be generally made from the fact that the change in gradation is not uniform and the boundary contour is blurred.
4. Cracks:
①, hot cracks are irregular dark junctions, which are usually broken lines and bifurcate, the gray value is smaller than that of the target entity, the broken lines on the ICT slice are fuzzy, the shape is complex, and the broken lines are not easy to identify;
②, cold crack, smooth line or curved smooth line, the gray value is smaller than that of the target entity, and the broken line is less clear.
Common methods used in the art for inspecting engine blades include fluoroscopic techniques, X-ray techniques, and ultrasonic pulse echoes, which require technical manual interpretation methods that are time consuming and labor intensive. Common methods also include two-dimensional image-based defect detection methods, including threshold-based methods, edge-detection-based methods, cluster-based methods, digital subtraction-based methods, and statistical-based methods. The method is easy to have the problems of shielding, shadow and the like in the measuring process, and the detection effect is influenced.
Disclosure of Invention
Aiming at the defects in the prior art, the engine blade measuring method provided by the invention can realize accurate measurement of the surface shape of the blade of the aero-engine.
In order to achieve the purpose of the invention, the invention adopts the technical scheme that:
an engine blade measuring method is provided, which comprises the following steps:
s1, constructing a measurement system based on structured light, scanning the engine blade through structured light projection, and acquiring a stripe pattern obtained through scanning;
s2, carrying out two-dimensional S transformation on the fringe pattern to obtain a two-dimensional S transformation coefficient;
s3, extracting two-dimensional S transformation coefficients at different frequencies at the same position, and recombining the two-dimensional S transformation coefficients into a matrix to obtain the frequency spectrum distribution at the position;
s4, acquiring the distribution of the scheduling values of the whole fringe pattern according to the frequency spectrum distribution of each position;
s5, reconstructing a three-dimensional surface shape of the engine blade according to the mapping relation between the distribution of the scheduling values and the height values of the whole fringe pattern;
and S6, measuring the engine blade by detecting the three-dimensional surface shape of the engine blade.
Further, the specific method of step S1 includes the following sub-steps:
s1-1, establishing a measuring system: the device installation is carried out according to the sequence of the structured light source, the condenser lens, the grating, the projection lens, the semi-transparent and semi-reflective mirror and the engine blade, a CCD camera is installed in the reflection direction of the semi-transparent and semi-reflective mirror, and the CCD camera is connected with an upper computer;
and S1-2, moving the grating at equal intervals in the scanning direction, and completing grating projection on the optical axis perpendicular to the projection and image acquisition of the CCD camera to obtain a fringe pattern.
Further, the structured light source generates a light field through a binary coding template, and a sinusoidal structured light field for structured illumination is obtained.
Further, the binary coding template comprises a one-dimensional error diffusion sine template, a two-dimensional error diffusion sine template and a binary area modulation sine template.
Further, the expression of the bar graph in step S1-2 is:
wherein I (x, y; delta) is a fringe pattern with the distance delta from the image acquisition plane to the image plane at the position of (x, y); r (x, y) is the reflectivity of the measured object surface; m is the transverse magnification of the measuring system; c0(x, y) is the contrast of the fringes; e is a natural constant; f. of0Is the frequency of the grating; sigmaHIs the diffusion constant; phi0(x, y) is the initial phase of the grating; i is0Is the background light field within the window footprint.
Further, the specific method of step S2 is:
according to the formula:
performing two-dimensional S transformation on the fringe pattern to obtain two-dimensional S transformation coefficients S (u, v, f)u,fv) (ii) a Wherein h (x, y) is a fringe pattern; pi is a constant; f. ofuAnd fvAs a frequency variable, following the position (u, v); coordinate variables u and v respectively represent the central positions of the two-dimensional Gaussian window on the x axis and the y axis; exp (·) is an exponential function with a natural constant e as the base; (x, y) is the position of the fringe pattern corresponding to the measured object; i is an imaginary unit.
Further, the specific method of step S4 is:
taking a ridge of the two-dimensional S-shaped transform spectrum at a certain position as the maximum value of the frequency spectrum distribution at the position, taking the absolute value of the ridge as the modulation value at the position, and traversing each position to obtain the scheduling value distribution of the whole fringe pattern; wherein the absolute value at the ridge is calculated as:
r (x, y) in the calculation formula is the reflectivity of the surface of the measured object; e is a natural constant; f. of0Is the frequency of the grating; sigmaHIs the diffusion constant; m0(x, y) is the lateral magnification of the measurement system.
Further, in step S5, the method for obtaining the mapping relationship between the scheduling value distribution and the height value of the whole fringe pattern includes:
and measuring a standard reference plane through a measuring system, and mapping the measured scheduling value distribution result with the corresponding height value to obtain the mapping relation between the scheduling value distribution and the height value.
The invention has the beneficial effects that: the projection optical axis of the invention is superposed with the observation optical axis, namely the projection direction of the grating is consistent with the direction of the stripes obtained by the CCD camera, the depth information of the measured object is coded in the fuzzy degree of the stripes, and the depth information of the measured object is obtained according to the change of the modulation degree of the stripes on the projection direction axis. When the projection optical axis and the detector optical axis are coincident, due to defocusing, the modulation degree of the stripes is modulated in the depth direction of the surface of the measured object in the direction of the projection optical axis, the three-dimensional surface shape of the measured object is reconstructed according to the corresponding relation between the distribution of the modulation degree and the depth information, and the corresponding engine blade can be detected by detecting the visual three-dimensional surface shape. The method can reconstruct the three-dimensional surface shape of the measured object without phase expansion, thereby not only avoiding the problems of shadow and shielding, but also avoiding the phenomenon of possible discontinuity in the phase expansion process, and realizing the accurate measurement of the surface shape of the blade of the aero-engine.
Drawings
FIG. 1 is a schematic flow diagram of the process;
fig. 2 is a schematic structural diagram of the measurement system.
Detailed Description
The following description of the embodiments of the present invention is provided to facilitate the understanding of the present invention by those skilled in the art, but it should be understood that the present invention is not limited to the scope of the embodiments, and it will be apparent to those skilled in the art that various changes may be made without departing from the spirit and scope of the invention as defined and defined in the appended claims, and all matters produced by the invention using the inventive concept are protected.
As shown in FIG. 1, the engine blade measuring method comprises the following steps:
s1, constructing a measurement system based on structured light, scanning the engine blade through structured light projection, and acquiring a stripe pattern obtained through scanning;
s2, carrying out two-dimensional S transformation on the fringe pattern to obtain a two-dimensional S transformation coefficient;
s3, extracting two-dimensional S transformation coefficients at different frequencies at the same position, and recombining the two-dimensional S transformation coefficients into a matrix to obtain the frequency spectrum distribution at the position;
s4, acquiring the distribution of the scheduling values of the whole fringe pattern according to the frequency spectrum distribution of each position;
s5, reconstructing a three-dimensional surface shape of the engine blade according to the mapping relation between the distribution of the scheduling values and the height values of the whole fringe pattern;
and S6, measuring the engine blade by detecting the three-dimensional surface shape of the engine blade.
The specific method of step S1 includes the following substeps:
s1-1, establishing a measuring system: as shown in fig. 2, the device installation is performed according to the sequence of the structured light source, the condenser lens, the grating, the projection lens, the half mirror and the engine blade, and the CCD camera is installed in the reflection direction of the half mirror and connected with the upper computer;
and S1-2, moving the grating at equal intervals in the scanning direction, and completing grating projection on the optical axis perpendicular to the projection and image acquisition of the CCD camera to obtain a fringe pattern.
The structured light source generates a light field through a binary coding template to obtain a sinusoidal structured light field for structured illumination. The binary coding template comprises a one-dimensional error diffusion sine template, a two-dimensional error diffusion sine template and a binary area modulation sine template.
The expression of the stripe pattern in step S1-2 is:
wherein I (x, y; delta) is a fringe pattern with the distance delta from the image acquisition plane to the image plane at the position of (x, y); r (x, y) is the reflectivity of the measured object surface; m is the transverse magnification of the measuring system; c0(x, y) is the contrast of the fringes; e is a natural constant; f. of0Is the frequency of the grating; sigmaHIs the diffusion constant; phi0(x, y) is the initial phase of the grating; i is0Is the background light field within the window footprint.
The specific method of step S2 is: according to the formula:
performing two-dimensional S transformation on the fringe pattern to obtain two-dimensional S transformation coefficients S (u, v, f)u,fv) (ii) a Wherein h (x, y) is a fringe pattern; pi is a constant; f. ofuAnd fvAs a frequency variable, following the position (u, v); coordinate variables u and v respectively represent the central positions of the two-dimensional Gaussian window on the x axis and the y axis; exp (·) is an exponential function with a natural constant e as the base; (x, y) is the position of the fringe pattern corresponding to the measured object; i is an imaginary unit.
The specific method of step S4 is: taking a ridge of the two-dimensional S-shaped transform spectrum at a certain position as the maximum value of the frequency spectrum distribution at the position, taking the absolute value of the ridge as the modulation value at the position, and traversing each position to obtain the scheduling value distribution of the whole fringe pattern; wherein the absolute value at the ridge is calculated as:
r (x, y) in the calculation formula is the reflectivity of the surface of the measured object; e is a natural constant; f. of0Is the frequency of the grating; sigmaHIs the diffusion constant; m0(x, y) is the lateral magnification of the measurement system.
The method for obtaining the mapping relationship between the distribution of the scheduling values and the height values of the whole fringe pattern in step S5 includes: and measuring a standard reference plane through a measuring system, and mapping the measured scheduling value distribution result with the corresponding height value to obtain the mapping relation between the scheduling value distribution and the height value.
In an embodiment of the invention, because the grating coding mode and the grating periodicity have important influence on modulation detection errors, the optimal grating coding mode and periodicity can be finally determined through theoretical analysis and experimental verification according to the size of the blade to be detected.
The sinusoidal grating is imaged by the projection lens, the fringe pattern collected on the focal plane is clearest, and the modulation value is also maximum. Due to defocusing, fringe patterns acquired in front of and behind the focal plane are gradually blurred, and the modulation value is correspondingly reduced. If the same-name pixel points of the fringe atlas collected in a certain range in front of and behind the focal plane are taken, the modulation degree distribution corresponding to the series of fringe atlas is in an inverted U shape, and the height corresponds to the maximum value of the modulation degree one by one, so that the mapping relation between the modulation degree and the height can be established, and the three-dimensional surface shape of the blade is finally reconstructed.
In the specific implementation process, a Zhangyingyou multi-plane calibration method can be adopted, known calibration objects are photographed, and the image coordinates of the feature points and the actual coordinates of the feature points in the calibration objects are extracted, so that the internal and external parameters of the camera are calculated. The coordinates of the system in the depth direction of the measurement space can be measured with a fine control translation rail and a standard reference plane. The software module realizes automatic translation control, image acquisition, modulation degree extraction and calibration calculation. And obtaining a coordinate interpolation lookup table in the measurement space after calibration for three-dimensional measurement of the blade of the aircraft engine.
In summary, the projection optical axis of the present invention coincides with the observation optical axis, that is, the projection direction of the grating coincides with the direction of the fringe acquired by the CCD camera, the depth information of the object to be measured is encoded in the degree of blurring of the fringe, and the depth information of the object to be measured is acquired according to the change of the modulation degree of the fringe on the projection direction axis. When the projection optical axis and the detector optical axis are coincident, due to defocusing, the modulation degree of the stripes is modulated in the depth direction of the surface of the measured object in the direction of the projection optical axis, the three-dimensional surface shape of the measured object is reconstructed according to the corresponding relation between the distribution of the modulation degree and the depth information, and the corresponding engine blade can be detected by detecting the visual three-dimensional surface shape. The method can reconstruct the three-dimensional surface shape of the measured object without phase expansion, thereby not only avoiding the problems of shadow and shielding, but also avoiding the phenomenon of possible discontinuity in the phase expansion process, and realizing the accurate measurement of the surface shape of the blade of the aero-engine.
Claims (8)
1. An engine blade measurement method, characterized by comprising the steps of:
s1, constructing a measurement system based on structured light, scanning the engine blade through structured light projection, and acquiring a stripe pattern obtained through scanning;
s2, carrying out two-dimensional S transformation on the fringe pattern to obtain a two-dimensional S transformation coefficient;
s3, extracting two-dimensional S transformation coefficients at different frequencies at the same position, and recombining the two-dimensional S transformation coefficients into a matrix to obtain the frequency spectrum distribution at the position;
s4, acquiring the distribution of the scheduling values of the whole fringe pattern according to the frequency spectrum distribution of each position;
s5, reconstructing a three-dimensional surface shape of the engine blade according to the mapping relation between the distribution of the scheduling values and the height values of the whole fringe pattern;
and S6, measuring the engine blade by detecting the three-dimensional surface shape of the engine blade.
2. The engine blade measuring method according to claim 1, wherein the specific method of step S1 includes the substeps of:
s1-1, establishing a measuring system: the device installation is carried out according to the sequence of the structured light source, the condenser lens, the grating, the projection lens, the semi-transparent and semi-reflective mirror and the engine blade, a CCD camera is installed in the reflection direction of the semi-transparent and semi-reflective mirror, and the CCD camera is connected with an upper computer;
and S1-2, moving the grating at equal intervals in the scanning direction, and completing grating projection on the optical axis perpendicular to the projection and image acquisition of the CCD camera to obtain a fringe pattern.
3. The engine blade measurement method of claim 2, wherein the structured light source generates a light field via a binary coded template, resulting in a sinusoidal structured light field for structured illumination.
4. The engine blade measurement method of claim 3, wherein the binary coded templates comprise one-dimensional error diffusion sine templates, two-dimensional error diffusion sine templates, and binary area modulation sine templates.
5. The engine blade measuring method according to claim 2, wherein the expression of the strip chart in the step S1-2 is:
wherein I (x, y; delta) is a fringe pattern with the distance delta from the image acquisition plane to the image plane at the position of (x, y); r (x, y) is the reflectivity of the measured object surface; m is the transverse magnification of the measuring system; c0(x, y) is the contrast of the fringes; e is a natural constant; f. of0Is the frequency of the grating; sigmaHIs the diffusion constant; phi0(x, y) is the initial phase of the grating; i is0Is the background light field within the window footprint.
6. The engine blade measuring method according to claim 1, wherein the specific method of step S2 is:
according to the formula:
performing two-dimensional S transformation on the fringe pattern to obtain two-dimensional S transformation coefficients S (u, v, f)u,fv) (ii) a Wherein h (x, y) is a fringe pattern; pi is a constant; f. ofuAnd fvAs a frequency variable, following the position (u, v); coordinate variables u and v respectively represent the central positions of the two-dimensional Gaussian window on the x axis and the y axis; exp (·) is an exponential function with a natural constant e as the base; (x, y) is the position of the fringe pattern corresponding to the measured object; i is an imaginary unit.
7. The engine blade measuring method according to claim 1, wherein the specific method of step S4 is:
taking a ridge of the two-dimensional S-shaped transform spectrum at a certain position as the maximum value of the frequency spectrum distribution at the position, taking the absolute value of the ridge as the modulation value at the position, and traversing each position to obtain the scheduling value distribution of the whole fringe pattern; wherein the absolute value at the ridge is calculated as:
r (x, y) in the calculation formula is the reflectivity of the surface of the measured object; e is a natural constant; f. of0Is the frequency of the grating; sigmaHIs the diffusion constant; m0(x, y) is the lateral magnification of the measurement system.
8. The engine blade measuring method according to claim 1, wherein the mapping relationship between the distribution of the scheduling values and the height values of the entire fringe pattern in step S5 is obtained by:
and measuring a standard reference plane through a measuring system, and mapping the measured scheduling value distribution result with the corresponding height value to obtain the mapping relation between the scheduling value distribution and the height value.
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